The present invention relates to a fuel-burner apparatus and method wherein a fuel is burned in an oxidant to heat a furnace heat-load, such as glass, ferrous and non-ferrous melts and etc. More particularly, the present invention relates to a fuel-burner apparatus and method involving globally-enhanced mixing of the oxidant and fuel.
Furnaces used in heating thermal loads such as glass and metal melts typically incorporate one or more burners set within burner blocks along the sides of the furnace. The burner produces the required heat by burning a liquid fuel, such as No. 2 or No. 6 fuel oil or a gaseous fuel such as natural gas in an oxidant such as oxygen or oxygen-enriched air. The resultant flame extends over the melt and heat is transferred from the flame to the melt by radiation and conduction.
Global-enhancement burners are provided in which the mixing of the oxidant and the fuel occurs over a large area as opposed to a localized mixing of the oxidant and fuel. As a result, a broad flame is produced having a controlled heat release pattern which can be quite uniform throughout the flame. An example of a global enhancement burner can be found in U.S. Pat. No. 4,927,357, in which a non-axisymmetric oxidant nozzle is located below a fuel nozzle to produce a low-pressure field of the oxidant below the fuel nozzle. The low-pressure field enhances aspiration of the fuel into the oxidant. The oxidant and fuel jets produced by the oxidant and fuel nozzles fan out from the burner so that the mixing between the two occurs over a wide area. The resultant flame produced by combustion of the fuel within the oxidant has quite a uniform heat distribution with the virtual elimination of hot spots. In some operating regimes, a long flame is produced in which unburned particles of fuel become increasingly more buoyant along the length of the flame. The disadvantage of this is that unburned particles of fuel at the end of the flame rise to burn outside of the oxidant provided directly through the burner in a controlled manner. This is typically observed as the flame licking up at its end. As a result, part of the heat released by the flame is diverted from the heat-load to the top or crown of the furnace.
Another disadvantage of many prior art burners, including global-enhancement burners, is that it is difficult to control the mode of heat transfer to the melt without changing the stoichiometry of the flame. In this regard, certain types of melts are highly reflective of radiant heat. In such case, it is known that more effective heat transfer can be obtained with a convective-type flame. One way to achieve this is to increase the velocity of the oxidant jet and thereby sharpen the flame pattern from a lazy flame pattern. A sharp flame results in a lower degree of radiative and a higher degree of convective heat transfer than a lazy flame. However, it is difficult to control the oxidant velocity independently of oxidant mass flow rate without a sophisticated flow-control system. As such, an increase in oxidant velocity is accompanied by a decrease in oxidant mass flow-rate. The decreased oxidant mass-flow rate changes the stoichiometry of the reaction between the fuel and the oxidant to in turn, change the rate at which heat is released by the flame and may result in unburned fuel in the exhaust system of the furnace.
As will be discussed, the present invention provides a burner that more effectively aspirates the fuel into the oxidant to prevent the more buoyant particles of fuel from burning outside of the oxidant. Additionally, a burner of the present invention allows for the velocity of the oxidant to be controlled independently of its mass flow rate to selectively produce either sharp or lazy flame patterns without affecting the stoichiometry of the reaction between the fuel and oxidant. As a result, the heat release characteristics of the flame can be adjusted from radiation dominated to convection dominated independently of stochiometry.